It is well-known that LIDAR can be used for measuring a velocity of a particle. Typically, a single beam-focusing optical unit, such as a telescope, is used for measuring the velocity. The single telescope is typically both used as a transmitter and a receiver of light, the light typically being a laser beam. First, light is transmitted to a target via the telescope, secondly, light is scattered at the target, and finally, backscattered light is received via the telescope, such that a velocity component can be determined.
A LIDAR system having a single telescope is limited to line of sight measurement, prohibiting the determination of, for example, a wind field having multiple velocity components. One solution to determine multiple velocity components, i.e. a plurality of velocity vectors, is to implement a scanning unit in the telescope, such that a wider field of view can be obtained. Another solution is to use multiple telescopes, and for example measuring wind speed components simultaneously, for example by splitting up a single light beam. The drawback of the latter solution implies a drop of transmitted optical power. Thus, it has been suggested to switch a single light beam between a plurality of beam-focusing optical units, in particular such that the beam-focusing optical units are pointed at the same probe volume, such that different views of the same target volume is provided.
In order to switch the light beam between a plurality of beam-focusing optical units, it has specifically been proposed to use fibre optics, such that the beam is switched and coupled into the beam-focusing optical units with little loss in optical power. Using fibre optics provides first of all a fixed configuration of the light and a specific transmission into for example the plurality of beam-focusing optical units. In this regard, a fibre optic solution is a very stable system. On the other hand, such a solution is also very sensitive to the configuration, or rather the alignment, of the beam-focusing optical units. For example, in assembly of LIDAR systems, the beam-focusing optical units can easily be misaligned, and the fibre optics may then be adjusted to adapt to the misalignment of the beam-focusing optical units. Alternatively, the beam-focusing optical units may be adjusted to the optical fibre, or a combination of the two methods, may provide an overall aligned LIDAR system. Such an alignment is cumbersome and time demanding. Further, if the LIDAR system is installed, for example on top of a wind turbine, it may not be easy to align either the fibre optics or the beam-focusing optical units, and there is therefore a need for a LIDAR system that provides easy and simple alignment of the beam.